There are many situations where is it desirable to locate buried utilities such as pipes and cables. For example, prior to starting any new construction that involves excavation, it is important to locate existing underground utilities such as underground power lines, gas lines, phone lines, fiber optic cable conduits, CATV cables, sprinkler control wiring, water pipes, sewer pipes, etc., collectively and individually referred to herein with the term “objects.” As used herein the term “buried” refers not only to objects below the surface of the ground, but in addition, to objects located inside walls, between floors in multi-story buildings or cast into concrete slabs, etc. If a back hoe or other excavation equipment hits a high voltage line or a gas line, serious injury and property damage can result. Severing water mains and sewer lines leads to messy cleanups. The destruction of power and data cables can seriously disrupt the comfort and convenience of residents and cost businesses huge financial losses.
Buried objects can be located by sensing an electromagnetic signal emitted by the same. Some cables such as power lines are already energized and emit their own long cylindrical electromagnetic field. Other conductive lines need to be energized with an outside electrical source having a frequency typically in a range of approximately 50 Hz to 500 kHz in order to be located. Location of buried long conductors is often referred to as “line tracing.”
A sonde (also called a transmitter, beacon or duct probe) typically includes a coil of wire wrapped around a ferromagnetic core. The coil is energized with a standard electrical source at a desired frequency, typically in a range of approximately 50 Hz to 500 kHz. The sonde can be attached to a push cable or line or it may be self-contained so that it can be flushed. A sonde generates a more complex electromagnetic field than that produced by an energized line. However, a sonde can be localized to a single point. A typical low frequency sonde does not strongly couple to other objects and thereby produce complex interfering fields that can occur during tracing. The term “buried objects” as used herein also includes sondes and marker balls.
Besides locating buried objects prior to excavation, it is further desirable to be able to determine their depth. This is generally done by measuring the difference in field strength at two locations.
The prior art includes many battery powered portable sonde and line locators that employ antennas to sense an electromagnetic signal emitted by buried objects and indicate their location via audible tones and displays. Those that have been commercialized have been difficult to use primarily because they are extremely sensitive to the orientation of their antennas relative to the buried object. With commercially available sonde and line locators it is possible to have signal strength go up as the operator moves farther away from the buried object. Thus these locators can indicate a peak, then a null and then a smaller peak. This can confuse the operator, especially if he or she interprets a smaller peak as the buried object. Users of sonde and line locators refer to the smaller peak as a ghost or a false peak. FIG. 1 is a graphical vertical sectional view that illustrates the foregoing difficulty. A sonde 10 is located inside a plastic pipe 12 beneath a concrete slab 14. The electromagnetic dipole field emitted by the sonde 10 is illustrated by concentric ovals 16. A conventional locator will “see” two smaller false peaks 18 and 20 spaced from the true larger peak 22 by a pair of nulls 24 and 26.
Conventional battery powered portable sonde and line locators have also suffered from user interfaces that are cumbersome to use, inflexible and/or limited in their ability to convey useful information. They typically have a small array of labeled push buttons and a display that is primarily dedicated to indicating numerical values in a manner that is not easy for the operator to interpret. Only a small number of commands can be executed in conventional sonde and line locators and the information is not displayed in a manner that intuitively indicates to the operator how close he or she is getting to the buried object.
There are many instances where the land that is to be excavated may be traversed or criss-crossed by several different utilities such as an AC cable, a water line, a gas line, a sewer pipe and a communications line. It would be desirable to be able to determine their paths and their depths all at one time. Conventional transmitters are commercially available that will output several different signals at different frequencies that can be applied to the same underground object or even to different underground objects, but the line locators that have heretofore been commercially available have not been capable of simultaneously detecting and indicating the locations of the different objects, their depths or their different types.